There is growing concern that Russia is using chemical weapons in Ukraine.
Use of chemical weapons is illegal and the Kremlin told the UN's Organisation for the Prohibition of Chemical Weapons in 2017 that it had destroyed its entire stockpile - 40,000 metric tonnes.
A year later, nerve agent novichok was used in an attempt to kill Russian defector Sergei Skripal in Salisbury.
Mr Skripal and his daughter survived.
Russia was widely condemned, despite denying involvement.
What sort of chemical weapons could President Putin possess?
Chemical weapons expert Hamish de Bretton-Gordon told Forces News: "Basically you have nerve agents, things like novichok or sarin, which attack the nerves and kill people very quickly.
"Then we have things like choking agents, like chlorine, which affect the respiration system, and we have blister agents which, as it suggests, create very nasty blisters on the skin - incapacitate people rather than kill them.
"On top of that, [there are] toxic industrial chemicals like ammonia and cyanide."
Watch: The Ukrainian women taking the fight to Russian forces.
Russia has already seen Bashar al Assad's regime deploy chlorine and blister agents in Syria.
The international community's tepid response to that attack could embolden the Russians in Ukraine.
Mr de Bretton-Gordon said: "When we had the massive nerve agent attack in Syria in August 2013, the so-called Obama red line didn't really mean very much, and the international community didn't react very strongly to it.
"[Russia] might well think that NATO won’t react this time.
"However, I think the Russians have been surprised and taken aback by the determined efforts of NATO to support Ukraine.
"The tactical advantage – one can see that fighting in towns and cities, successful in Syria.
"But now that we’re starting the major battles in the Donbas - very open areas - one can’t really see the utility of chemical weapons".
"Hopefully none of these will come to fruition."